11-02-2016, 04:19 PM
Why the exciting current of induction motor is so high as compared to the power transformer?
Unlike that of a power transformer, the magnetic circuit of the induction motor has an air-gap. Therefore, the exciting current of induction motor (3O to 4O% of full-load current) is much higher than that of the power transformer. Consequently, the exact equivalent circuit must be used for accurate results.
How transformation ratio of induction motor is different as compared to power transformer?
In a transformer, the windings are concentrated whereas in an induction motor, the windings are distributed. This affects the transformation ratio.
Why it is required to start 3-Phase Induction Motors?
The induction motor is fundamentally a transformer in which the stator is the primary and the rotor is short-circuited secondary. At starting, the voltage induced in the induction motor rotor is maximum ( s = 1). Since the rotor impedance is low, the rotor current is excessively large. This large rotor current is reflected in the stator because of transformer action. This results in high starting current (4 to 1O times the full-load current) in the stator at low power factor and consequently the value of starting torque is low. Because of the short duration, this value of large current does not harm the motor if the motor accelerates normally.
What is the effect of staring of induction motor on connected line?
Large starting current will produce large line-voltage drop. This will adversely affect the operation of other electrical equipment connected to the same lines. Therefore, it is desirable and necessary to reduce the magnitude of stator current at starting and several methods are available for this purpose.